The present invention relates to a spectrophotometer, comprising a housing in which a measuring system is arranged, which housing has a measuring opening, via which light is passed to the measuring system, and whereby the measuring system comprises a grating monochromator, an autocollimator cooperating therewith, and detection means for the light originating from the grating monochromator. More in particular, the invention relates to such a spectrophotometer, which also comprises an illuminator in a 45xc2x0/0xc2x0 configuration with a light source formed by a lamp and an illuminator optic, whereby after reflection light emitted by the lamp is passed via a measuring opening into a housing.
A spectrophotometer as defined in the opening paragraph is known from the international patent application WO82/0163. For such a measuring system a spectrophotometer ISO standards have been fixed; in the most recent ISO standards for color measurement an optical band width of 10 nm is recommended and a maximum value of 20 nm is prescribed. Moreover, the measuring system must be sensitive to collimated light to comply with these standards, whereby the rays entering the housing are allowed to deviate from the optical axis by up to 5xc2x0. The sensitivity to light from other directions must be minimal.
As indicated, light refraction takes place by means of an optical reflection grating. The detection often takes place by means of an array of photosensitive cells, in particular an integrated circuit with an array of photodiodes. In addition, other methods of light refraction and detection are used in practice. Known is, or instance, light refraction by means of a prism or a number of constant color filters or a linearly variable color filter. The detection may also occur with one single measuring cell, whereby for measuring different wavelengths the optical grating or the color filter device is rotated, for instance with a stepping motor with which the whole measuring range can be scanned within a few seconds.
Measuring techniques by means of grating monochromators are extensively described in E. G. Loewen, E. Popov; Diffraction gratings and applications (Marcel Dekker, Inc., New York, 1997), in particular in Chapter 12 thereof. Such techniques can be divided into two main categories: constructions with plane reflection gratings and constructions with concave reflection gratings. Nearly all the constructions with plane reflection gratings make use of one or more concave mirrors for collimating the light. A known exception is the so-called Littrow arrangement, which, however, is designated in the above literature as out-of-date (page 444). The concave reflection gratings are relatively expensive, but offer various advantages, in particular because of their applicability in the UV range and because of their simple construction. As a special advantage it holds that the functions of dispersion and autocollimation are combined therein; other optical components are not required therefor, which also prevents lining-up and stability problems.
For plane reflection gratings the Monk-Gillison arrangement is designated as the simplest and cheapest system (see the above literature, paragraph 12.5). In this system only two components are used for dispersion and collimation, namely a concave mirror and a reflection grating.
The object of the invention is to provide the spectrophotometer with a measuring system which means a further simplification with respect to the Monk-Gillison arrangement, which is additionally inexpensive and also complies with the above ISO standards.
According to the invention the spectrophotometer as defined in the preamble is characterized in that the grating monochromator and the autocollimator form a grating lens which, as a single, physical structure, has on one side convex collimator lens structure and on the other side an externally mirrored grating structure, the plane of the grating structure being inclined at a relative small angle to the optical axis of the measuring system and/or to the optical axis of the collimator lens. The edges of the grating lens are blackened to extinguish undesired reflections.
From U.S. Pat. No. 4,838,645 a reflecting diffraction grating is known in which a grating monochromator and an autocollimator which form one whole optical system, but not a single physical structure.
In the simplest form the grating lens is made of an optically bright plastic, preferably acrylate glass (PMMA). By using inexpensive manufacturing methods such as injection molding or pressing, the cost price of such a lens can be low. It is also possible to co-form fastening edges which facilitate the mounting, Also, the mirroring of the grating lens with aluminum evaporated under vacuum can be realized in an inexpensive manner in a mass production.
The grating structure can be made in a mold for the lens in the form of a so-called ruled grating. The original master grating can be notched with a diamond chisel in an optically pure plane substrate by means of a so-called ruling engine. Ruled gratings have a higher efficiency than holographically made gratings. By making the proper selection of the blaze angle, the efficiency can be optimized for the desired measuring range, in this case the visible light spectrum. This is an advantage over concave gratings, with which it is difficult to obtain a comparable efficiency.
In a preferred embodiment the measuring system comprises a Littrow arrangement whereby light enters the housing in a first direction (Y-direction) and falls therein on the grating lens via an entrance slit and a reflecting element in a direction substantially perpendicular thereto, the negative Z-direction, while reflected light from the grating lens substantially falls on the detection means in the positive Z-direction. The measuring system thereby depicts the slit on the detection means.
To keep the rays falling on and reflected by the grating lens separated, the angle therebetween in the YZ-plane is in the order of 15 to 20xc2x0. It will be bright that this angle value is only motivated by practical considerations. The optical axis of the lens curvature in the X-direction, perpendicular to the YZ-plane, falls into the YZ-plane, namely in the Z-direction. The plane of the grating is inclined at an angle in the order of 6xc2x0 to the XY-plane.
The detection means are formed by an array of photosensitive cells, the dimensions of which are in the order of 0.2xc3x970.2 mm or less, while, furthermore, a cylindrical lens is present to converge the light from the grating lens on the photocells. The entrance slit extending in the Z-direction often has a size of about 2 mm, while the width thereof extending in the X-direction is about 0.2 mm, which corresponds with a band width of about 10 nm, so that, when the dimensions of the photocells is of the same order as the size of the projected entrance slit, an array length of 6 to 9 mm is necessary to enable depicting of the visible spectrum. Arrays of narrow long photocells in the size of the above slit are certainly manufactured, but they are considerably more expensive than the more conventional arrays, the cells of which have a smaller and substantially square cross-section, often in the order of 0.1xc3x970.1 mm. For reasons of cost price, it is favorable to use these arrays, but the light-sensitivity of such small photocells is often lower. To compensate this drawback, the array in the device according to the invention is provided with a cylindrical lens which converges the about 2 mm high picture on the small photocells, which causes the local light intensity to be increased proportionally. Because of the small dimensions of the array the focal distance of the grating lens may also be small and thus the whole optical system. The whole housing of the measuring system can therefore be kept within the dimensions of 3xc3x973xc3x975 cm.
Scattered light and radiation of higher orders must be reduced or stopped in the conventional manner by providing stop filters in the optical path. The position of such stop filters, which have to act on a part of the useful spectrum, is just before at least part of the photocells. These filters can, for instance, be cemented to the cylindrical lens, thereby preventing additional surface reflections. An infrared stop filter may be arranged at the entrance of the system to prevent infrared scattered light to which silicon photodiodes are very sensitive. This filter is of such quality that the whole desired spectrum is sufficiently passed. Moreover, at a number of places in the housing of the measuring system plates are arranged which only leave an opening for the desired light bundle and thus form chambers in which scattered light extinguishes. This is in particular important to separate white light reflections.
In the simplest embodiment of the grating lens this lens is symmetrical with respect to rotation, the axis being directed between the virtual position of the slit and the middle of the array. A part of the light falling on the grating lens, at least 4%, will not pass through the lens part thereof to be mirrored back after diffraction, but already be mirrored back on the surface as white light; such reflections are called Fresnel reflections. Because a part of the white light is mirrored to the array, measuring errors arise. These are even intensified because the energy of the white light is much higher than that of the monochromatic light which is nominally measured by the photocells. In a preferred embodiment of the grating lens this is prevented by an xe2x80x9coff-axisxe2x80x9d design. The lens is, as it were, placed obliquely backwards, so that this scattered light reflects back at such an angle that it does not reach the array. This oblique placement causes considerable changes in the optical action of the lens. Because of the asymmetry, more image formation (coma) appears which adversely affects the optical band width of the measurement. Also, the angle of the grating with respect to the Y-axis in the XY-plane must be changed.
In a further improved embodiment the curvature of the lens in the Y-direction is made less strong than in the X-direction. The lens thus obtains a toric shape. It has been found that at a proper selection of the curvature in the Y-direction the optical deformation is strongly decreased, while the lens still sufficiently converges in the Y-direction. The cylindrical lens for the array needs to be only slightly magnified.
In yet another embodiment the lens is cylindrically shaped with only a curvature in the X-direction. In that case the image formation can become even smaller in the inclined position required to prevent Fresnel reflections. The picture of the entrance slit, however, thereby becomes even larger in the Y-direction, which may lead to an impractically large cylindrical lens or to a loss of sensitivity.
Furthermore, it proves to be also possible to inhibit Fresnel reflections when the collimator lens structure, that is to say the grating lens, is designed as one having a symmetric or slightly asymmetric toric shape, with a damping element being arranged in or near the middle.
The monochromator is often defined from the entrance slit. The admissible opening angle determines the light intensity of the system. Conventional is a numerical aperture of slightly larger than 0.1, which is half an opening angle of 6 to 8xc2x0. At a larger lens opening too much deformation occurs. In the grating lens according to the invention the optical deformation at a large aperture increases even more strongly than in the known multi-element systems. However, for use as a colorimeter this is no drawback or limitation because ISO standards for reflection measurement prescribe a small aperture. The half opening angle may be at most 5xc2x0.
In surface color measurement it is conventional to measure an approximately circular surface having a diameter of a few mm and not having a relatively long narrow slit. The testing targets are often small square color areas. In this optical system a good adaptation to an approximately round measuring opening can be obtained by making use of a lens which converges collimated light from the measuring opening in the X-direction on the entrance slit, while the lens contrarily diverges in the Y-direction through the longitudinal direction of the slit until at the total height of the grating lens. Therefore, in a concrete embodiment the measuring opening in the housing is substantially circular and between this measuring opening and the entrance slit a saddle-shaped lens is present which converges collimated light from the measuring opening in the X-direction, perpendicular to the YZ-plane, on the entrance slit, while this lens diverges the light reflected by the reflecting element in the Y-direction. Such a lens shape may also be inexpensively produced in plastic, for instance by an injection molding technique. Preferably, saddle-shaped lens is, in particular on the plane side, provided with an infrared stop filter, so that no infrared energy enters through the entrance slit into the housing of the measuring system.
Because the elements of the array of photosensitive elements are sensitive to various wavelength intervals in different ways, one or more blocking filters adjusted to different wavelength intervals may be arranged before the array of photosensitive elements. Very accurate filters are obtained in the form of an integrally formed blocking filter combination of filters secured against each other, the transition area extending obliquely in two directions X and Y or the transition areas extending obliquely parallel to each other in two directions X and Y. Here a transition area may be inclined in the X direction at an angle xcex2 of 10 to 70xc2x0, preferably of 10 to 40xc2x0, in particular approximately 20xc2x0, and in the Y direction at an angle xcex1 of 10 to 70xc2x0, preferably 30 to 60xc2x0, in particular 45xc2x0.
The spectrometer according to the invention is an inexpensive, handy and reliable instrument for measuring surface colors of printing matter, paint, plastics, textiles, foodstuffs, etc. In particular without illuminator the spectrophotometer may serve for measuring light sources, for instance for displays, for inspection of theater and studio lighting, office and street lighting. This spectrophotometer is in particular suitable for connection to a computer in which the information of the detection means can be processed.